div{
    color: white;
    height: 300px;
    width: 300px;
}

div:nth-child(2n+1){
    background-color: #0000FF;
}

div:nth-child(2n + 2){
    background-color: rgba(255,0,0,0.7);
}

div.small-div{
    width: 200px;
    height: 200px;
}

div #big-div{
    width: 400px;
    height: 400px;
}

div.hover-div:hover{
    background-color: orange;
}